Is Tar Paper or Housewrap needed under new siding?

I am going to install some new wood siding on our garage. Do I need to put tar paper, or some other house wrap, underneath the siding? I have heard that some builders don't use anything. What's best? Thanks.

Ben C. ~ Albany, NY

Thanks for your question Ben, and I'm not surprised that you have heard differing opinions on which type of house wrap is the right product to install on your garage. In fact, many builders disagree on the effectiveness of house wraps altogether, and while some builders will only install 15-pound asphalt-saturated felt paper underneath exterior siding, others swear by plastic-fabric house wraps, such as Typar or Tyvek.

In general, I think that installing some sort of weather barrier underneath your garage's siding is essential. Not only will this barrier protect the plywood(or OSB) sheathing from any rain water that may find its way underneath your new siding, a properly installed house wrap can also improve the thermal performance of the building by helping to prevent outside air from infiltrating the wall cavity. Preventing air infiltration also helps to keep moisture from entering the structure, because sometimes the outside air is laden with water vapor.

One of the biggest advantages of using asphalt-saturated felt is that is able to self-seal all of the little holes caused by staples and nails during the installation process. That being said, I find that felt tears too easily and becomes very brittle during a cold weather installation. It is also much heavier than house wraps that are made from thin, light weight plastic fabrics, such as polyethylene(Tyvek) and polypropylene(Typar). Another problem with using asphalt-saturated felt is that it is difficult to seal the seams, unlike with plastic house wraps, which can be sealed with specialized tapes that are bombproof. I also think that the available flashing tapes that are needed around window and door openings seal better when used with the plastic house wraps instead of the asphalt-saturated felt.

My other concern with asphalt-saturated felt is that it does not breath as well as many of the available plastic house wraps, which can prevent moisture from getting out of your house. This means that moisture may collect on the underside of the felt, where it can damage the exterior sheathing of the building. On the other hand, plastic house wraps are designed to work like a Gore-Tex raincoat, which repels water from the outside, but also allows moisture to escape from the inside. This is why I don't use felt, and generally choose Typar.

Whichever product you choose, make sure that you follow the manufacturer's installation instructions carefully. All to often, well-designed building products are installed incorrectly, which reduces their effectiveness and usually causes some other unforeseen problem down the road. Good luck with your project, and I hope your garage looks like new once the siding is installed!